Analysis of Nitrogen Loading Reductions for Wastewater Treatment Facilities
and Non-Point Sources in the Great Bay Estuary Watershed
Philip Trowbridge, P.E.N.H. Dept. of Environmental Services
Great Bay Estuary• Watershed covers
1023 mi2 (80% in NH, 20% in ME)
• Home to 14% of the population of NH and ME
• Contains 18 WWTFs
• Fed by 7 tidal rivers• Part of the EPA
National Estuary Program
Numeric Nutrient Criteria• The 2006 State of the
Estuary report by PREP showed signs of nutrient enrichment
• In 2009, DES developed numeric nutrient criteria to protect eelgrass habitat and prevent low dissolved oxygen in the estuary.
• Reviewed and validated by national experts
• Confirmed by recent studies
Nitrogen Impairments• Nutrient criteria
were used by DES to add most of the estuary to the 303d list for nitrogen impairments in 2009.
• The impairments triggered a TMDL process.
Estuary Assessment ZonesImpaired
Not Impaired
0 1 2 3 4 5 Miles
N
# Squamscott RiverSubestuary
#
Lamprey RiverSubestuary
#Winnicut RiverSubestuary
#
Little BaySubestuary
# Great BaySubestuary
#
Oyster RiverSubestuary
#
Bellamy RiverSubestuary
#
Cocheco RiverSubestuary
#
Salmon Falls RiverSubestuary
#
Upper Piscataqua RiverSubestuary
#
PortsmouthHarborSubestuary
#
Lower Piscataqua RiverSubestuary
#
Little Harbor/Back ChannelSubestuary
Note: The Assessment Zones on this mapcover both the NH and Maine portions of the Piscataqua River.
Nitrogen TMDL Process for the Great Bay Estuary
Completed – Great Bay Nitrogen Loading Analysis• Set preliminary nitrogen loading thresholds for
the seven watersheds• Determine options for bulk wasteload and load
allocations by watershedFuture Work• Determine non-point sources of nitrogen in each
watershed• Develop Watershed Implementation Plans and
TMDLs for each watershed based on detailed planning and local input
Questions to be answered by the Great Bay Nitrogen Loading Analysis
• What are the loading thresholds to meet the nutrient criteria and how much of reduction from existing loads would be needed?
• What would be the effects of different NPDES permitting scenarios for WWTFs on nitrogen loads and requirements for NPS reductions?
• How much will WWTF upgrades cost?
Methods for the Great Bay Nitrogen Loading Analysis
• Current Watershed Nitrogen Loads• Watershed Nitrogen Loading Thresholds• Evaluation of Permitting Scenarios• Quality Assurance• WWTF Capital and O&M costs
The study was limited to watershed upstream of Great Bay, Little Bay, and Upper Piscataqua. The model could not be applied to the Lower Piscataqua or Portsmouth Harbor.
Study Area
Mass Balance Model for Watershed Nitrogen Loading Thresholds
Sources of NitrogenWWTFs in watershedWWTFs direct dischargeWWTFs in Lower PiscataquaNPS upstream of damNPS downstream of damGroundwaterAtmospheric Deposition
Total N Load “Ltot” (mg/s)
Sources of WaterWWTFs direct dischargeStreamflow above damRunoff below damGroundwaterPrecipitation to surfaceNet loss from withdrawalsOcean (tidal flushing)
Total Flow “Qtot” (L/s)
From WatershedUpstream of Dam
To/FromOcean
Groundwater And Runoff from WatershedBelow Dam
Precipitation andAtm. Dep. To Water Surface
Example: LampreyRiver Subestuary
Ave N conc.In subestuary
Ave N conc. in subestuary from wshed sources
Ave N conc. in subestuary from ocean water
Different Types of Watershed Nitrogen Loading Thresholds
• Threshold for watershed loads to prevent DO violations locally
• Threshold to protect eelgrass locally– Use equations above substituting 0.3 mg/L for Ncrit
• Threshold to protect eelgrass in downstream subestuariesThreshold to protect eelgrass locally in downstream subestuary minusexpected N load from downstream WWTFs, divided between watershedsaccording to percent of existing upstream N load.
Margin ofSafety (0.9)
NutrientCriteria (0.45 mg/L)
Total WaterFlow
N conc. in ocean (0.2 mg/L)
Salinityin Estuary
Salinityin Ocean
N conc. in estuarydue to ocean water
Modeled Location:GRBLR Sonde
Selected Salinity:12 ppt (Sest)
Ocean Salinity:31.6 ppt (So)
Nitrogen concentration inestuary due to ocean waterNest|o= 0.2 mg/L *(12/31.6)
= 0.076 mg/L
Example: LampreyRiver Subestuary
Average Salinity atMonitoring Stations
in 2003-2004
Water Budget Value (cfs)WWTFs direct discharge 1.04Streamflow above dam 344.76Runoff below dam 2.77Groundwater 0.64Precipitation to surface 0.52Net loss from withdrawals -1.92Ocean (tidal flushing) 212.94
Total Flow “Qtot” 560.74 (cfs)15,880 (L/s)
To Prevent DO Violations = 0.9 x (0.45 – 0.076) x 15,880 = 5346 mg/s = 185 tons/yr
To Prevent Eelgrass Loss = 0.9 x (0.30 – 0.076) x 15,880 = 3202 mg/s = 111 tons/yr
Upstream watershed threshold to protect eelgrass in Great Bay = 330 tons/yrPercent of existing upstream load from the Lamprey watershed = 49.6%Downstream protective threshold = 330 x 0.496 = 163 tons/yr
Watershed Nitrogen Loading Thresholds for Local Effects
Watershed Nitrogen Loading Thresholds for Downstream Effects
TotalFreshwater Flow
Salinity in ocean
Salinity in estuary
Example: LampreyRiver Subestuary
2003-2004
Evaluation of WWTF Permitting Scenarios on Nitrogen Loads
• 33 scenarios• Calculate delivered N
load for scenarios• Compare delivered N
load to thresholds– Yellow: Prevents low
dissolved oxygen– Green: Protects local
eelgrass– Red: Protects
downstream eelgrass• Determine NPS
reductions for each WWTF permit scenario
Delivered Nitrogen Loads for the Lamprey River Subestuary for Permitting Scenarios, 2003-2004
Reductions in Nitrogen Loads for the Lamprey River Watershed
5% 41%24%
Non-Point Source Reductions for the Lamprey River Watershed
• WWTFs contribute 15% of the nitrogen to Lamprey River Subestuary.
• If WWTFs are permitted at…– 8 mg/L: NPS reduction of 17% or 38% needed– 5 mg/L: NPS reduction of 15% or 36% needed– 3 mg/L: NPS reduction of 13% or 34% needed
To protect eelgrass in downstream areas and prevent low DO in the tidal rivers
To protect eelgrass in all areas
Reductions in Nitrogen Loads for the Whole Watershed
30% 31% 45%
Non-Point Source Reductions for the Whole Watershed
• WWTFs contribute 27% of the nitrogen to Great Bay, Little Bay, and Upper Piscataqua.
• If WWTFs are permitted at…– 8 mg/L: NPS reduction of 32% or 50% needed– 5 mg/L: NPS reduction of 22% or 41% needed– 3 mg/L: NPS reduction of 16% or 34% needed
To protect eelgrass in downstream areas and prevent low DO in the tidal rivers
To protect eelgrass in all areas
Quality Assurance Tests• Accuracy
– Measured vs Modeled N concentration
– +/-11% (average)
• Precision– Monte Carlo Uncertainty
Analysis– +/-12% for tidal rivers,
+/-29% downstream areas
• Sensitivity– Monte Carlo Sensitivity
Analysis– Important variables are
ocean N conc., stream flow, salinity, criteria
Measured Modeled
A + B = C
+ =
Summary• Most of the Great Bay estuary is impaired for nitrogen as
shown by persistent low dissolved oxygen in the tributaries and eelgrass loss.
• Mass balance models predict that watershed nitrogen loads need to be reduced by 30-45%.
• Non-point source reductions range from 16-50% depending on the WWTF permitting scenario.
• Reducing nitrogen loads enough to prevent low dissolved oxygen in the tributaries will typically also protect eelgrass in downstream areas.
Next Steps• Identify non-point sources of nitrogen in the watershed
and reductions in the non-point source loads if best management practices are implemented.
• Develop models and nitrogen loading thresholds for the Lower Piscataqua River, Portsmouth Harbor, and Little Harbor.
• Continue research on nutrient criteria and existing models.
• Develop a comprehensive monitoring program to track the effectiveness of phased implementation activities.
• Develop Watershed Implementation Plans and TMDLs for each watershed.
For More InformationPhilip Trowbridge, P.E.N.H. Dept. of Environmental [email protected]
Reports available at:http://des.nh.gov/organization/divisions/water/wmb/coastal/great-bay-estuary.htm